Switching FBOs or Attachments Slow

Hi,

I'm doing a one off project that requires blurring and rendering to a bunch of FBOs before compositing for the final frame. Final target size is 4000x1000. I'm noticing that the combination of the frame size and switching FBOs is really slow. My loop looks something like this:

It seems at the full target size, the frame rate drops from 60 to 30. If I half the target size, the frame rate jumps back up to 60. One might expect this. However if, I just don't draw anything at all at the full target size and just have it bind and unbind, the frame rate is still 30.

I'm wondering if there is something I can do at full size to prevent the frame rate from just suddenly halving itself. My best guess is that it's causing some kind of stall or sync issue with in the card itself.